Near-to-Eye Displays (NEDs) utilizing diffractive Exit Pupil Expanders (EPEs) made on flat substrates are suitable for generation images of virtual displays at infinite viewing distance. If used for near-distance objects, each object point is replicated in the EPE plate, which is seen as pixel blurring in the virtual display. For example, if the imaging optics of the NED is designed to provide an input image at a finite viewing distance, each display pixel will consist of a range of ray angles. When coupled into a planar EPE the whole range of incident ray angles will emerge from all the ray-interception points along the out-coupling grating. Thus, a single display pixel will appear to be replicated or blurred when observed through the EPE. The limitation of infinite viewing distance may be a problem for near-sighted people. In particular, in mobile applications compact eye-glass type NEDs are preferred that are not well suited for wearing with regular eye glasses. Furthermore, the limitation of infinite image distance can lead to significant reduction in the viewing experience and limit the range of potential applications, such as stereoscopic 3D (3-dimensional) viewing.
The situation can be improved with the use of a cylindrically shaped EPE as described in the PCT patent application, International Publication Number WO2006064301. In such arrangement horizontal rays (in a direction perpendicular to an axis of a cylinder surface of the EPE substrate) emanated from the center of the object plane can be imaged perfectly, i.e., the object point replication along the horizontal direction as observed with the flat EPE plate is cancelled. However, rays with a vertical propagation component (in a direction parallel to the axis of the cylindrical surface) are shifted in a vertical direction and, thus, the object points will appear as lines when observed through the cylindrical EPE with substantial spreading. In general, the amount of vertical spreading of the object points depends on the focus distance (EPE curvature), vertical expansion, and pupil size of the observer.